The aim of this project is to provide the solutions to obtain accurate estimates of exclusion zones for marine mammals, in the context of seismic exploration, in a simple and quick way. Short timeframes available for noise mitigation studies during seismic surveys tend to result in incomplete or inaccurate results, which have a direct impact on the health of marine mammals at individual and population levels. The project aims at addressing this through a better understanding of a simple propagation model and through the development of an application for efficient calculation of the sound field and marine mammal exclusion zones.

Project outline

Marine seismic surveys are carried out to explore areas with potential for fossil fuels. They use high-power acoustic sources that produce impulsive, low-frequency sounds that can propagate long distances in the water. For a seismic contractor to acquire the permit to operate in a certain area, they must guarantee that the environmental impact is kept to a minimum and appropriate mitigation measures are adopted. One form of mitigation used to prevent noise impact on marine mammals consists in defining an exclusion zone, an area within which sound levels can cause temporary or permanent auditory damage. That zone is defined based on the sound field and the auditory response of vulnerable marine species, and is monitored in real time to ensure no marine mammals enter it during operations. A shutdown protocol is set in place for those situations.

During work at Seiche Ltd, the team travelled to Alaska to collect some audio and navigation data during a pre-planned operation of a seismic airgun array. For that purpose, they used six drift buoys that were deployed upstream in a tidal bay (Cook Inlet), and allowed to drift downstream to map the sound field around the source. The data, processed at the time for sound source verification (SSV) and exclusion zone estimation, will be reused in the PhD to better understand the propagation of seismic pulses in complex, dynamic underwater environments.

The project aims at providing a simple, quick and reliable way of estimating marine mammal exclusion zones. The three main challenges are:

1. Obtaining accurate sound field simulations. A thorough understanding of the source and the environment and the use of an appropriate propagation model are important factors when calculating sound levels. However, it is also important that the model provides quick results when sound field and exclusion zone simulations are requested. Developing an integrated modelling solution that comprises a source model, a propagation model and automatic environment definition, is one of the goals of this project.

2. Obtaining an effective measurement sample that can be used for model validation or direct estimation of exclusion zones. Operational restrictions exist for vessels or marine systems that are external to a seismic survey, which limit where and where measurements can be taken. The best sampling strategy will result in the least conflict with the seismic operations, while providing a reliable sample for model validation or exclusion zone calculation. The project aims to create a simple sound propagation model that will help to better understand underwater sound propagation and how different acoustic phenomena and patterns relate to operational and environmental parameters.

3. Sound propagation models require a certain degree of expertise, time and resources, often resulting in rough extrapolations of sound field measurements being reported. A simple sound propagation model would help bridge the gap and help companies or individuals that have limited expertise, time or resources to obtain reliable estimates of sound levels based on few measurements. The goal is that in the future published source levels and other extrapolated sound level values are comparable and usable.

Science

The specific tasks to tackle these challenges are:

• Code implementation of airgun array signature model, to be calibrated using parameter optimisation (simulated annealing) based on the most detailed database of controlled measurements of air gun signatures (Svein Vaage dataset).

• Verification of propagation model by calculating best-fit environmental parameters using an appropriate optimisation algorithm.

• Automated sound field simulation through simple specification of air gun array layout (i.e. automatic environment definition).

• Improved sampling strategy evaluated through the analysis of propagation loss patterns at various distance regions.

• Adaptation of image source model (i.e. simple propagation model) to aid in source level estimation from few measurements.

• Study of applicability limits of simple propagation model.

A conference proceeding was presented at the "Acoustic and Environmental Variability, Fluctuations and Coherence", Cambridge (UK), on the 12-13th 2016. The article was submitted to the Proceedings of the Institute of Acoustics (Ph. Blondel, G. Jimenez, R. Wyatt; “Seismic surveys in complex environments: effects of environmental variability on sound propagation and mitigation practice”, Proc. IOA, 38(3):16:23, 2016).

This is the PhD project of Guillermo Jimenez-Arranz. He is supervised by Philippe Blondel, Chris Budd, and Mark Burnett at Seiche Ltd.